Keying circuit



Jan. 7, 1936.

W. W. EITEL ET AL KEYING CIRCUIT Filed Feb. 27, 1934 INVENTORS,

WILL/AM w. E/TEL.

JACK MC cuLL'ouaH.

Patented Jan. 7, 1936 UNITED STATES PATENT OF'FWE KEYING CIRCUIT Nevada Application February 2'7, 1934, Serial No. 713,1tl7

1 Claim.

Our invention relates to thermionic tube oscillators, and more particularly to a means'and method of keying such an oscillator at signal frequency.

Among the objects of our invention are: to provide an improved method of keying an oscillator; to provide a means for keying a thermionic tubeconnected in a tuned-platetuned-grid oscillating circuit; and to provide a means and method 10 of keying an oscillator whereby only a small amount of power passes through the keying device.

Other objects of our invention will be apparent or will be specifically pointed out in the descripl5 tion forming a part of this specification, but we do not limit ourselves to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claim.

The figure is a diagrammatic representation,

reduced to lowest terms, of a vacuum tube oscillator and associated circuits, showing our preferred connection for keying. The circuit as shown also includes means for maintaining the heat dissipation of the tube substantially constant during the keying at signal frequency.

Other novel features of this circuit are described and claimed in our copending application entitled Oscillator system, Serial No. 713,106, filed February 2'7, 1934.

There are, in the art, various means and methods of keying vacuum tube oscillators. By keying we mean, in the present disclosure, the operation of setting the tube into oscillation and stopping it from oscillating, under control of the keying device.

We prefer to apply our invention to the well known tuned-plate tuned-grid circuit wherein a tuned circuit is connected between the grid and cathode, and another circuit, tuned to substantially the same frequency, connected between the cathode and anode. When such conditions are fulfilled these two circuits need not be coupled externally of the tube as the grid-to-plate capacity inside the tube is usually sufficient, and in many cases more than sufficient, to set the tube into eflicient oscillation. Such a circuit is stable and widely utilized in the art. Our present invention is directed toward an efficient method of keying an oscillator of this type.

In broad terms, our invention comprises a thermionic tube connected in a tuned-plate tunedgird circuit, the circuits being adjusted to produce oscillations in the'tube by virtue of the internal feed-back therein. We prefer to neutralize this internal feed-back to prevent the tube from oscillating, and then toprovide an external feed-back circuit linking the tunedplate circuit with the tuned-grid circuit, and so adjusted as to carry just sufiicient feed-back cur-' rent to cause the tube to oscillate properly. We prefer to insert a keying device in such a manner as to make and break the feed-back current. The tube is thereby set into oscillation or stopped in accordance with the key control of the feed-back current, and as this feed-back current is only a. small percentage of the output of the tube, large powers can be handled without excessive arcing at the key. contacts and without substantially affecting the stability of oscillation.

Our invention may be better understood by reference to the figure which illustrates a three electrode tube connected in a preferred tuned-plate tuned-grid circuit embodying our present invention, and the following description will also include our preferred method of maintaining heat dissipation substantially constant, as mentioned above.

A vacuum tube I having the conventional cathode 2, grid 3, and anode 4, is connected in a tuned-plate tuned-grid circuit comprising a grid oscillating circuit consisting of a grid inductance 5 and a parallel grid tuning condenser ti. One end of this tuned circuit is connected directly to the grid, the other end is connected through a by-pass condenser l to a leg 8 of the cathode 2. We prefer to ground the end of the inductance 5, through a condenser l3 and a lead 9. The tuned-plate circuit comprises a plate inductance Ill shunted by a plate tuning condenser H. One end of this inductance is connected to the plate and the other end is connected by a lead I? through a neutralizing condenser It to the grid 3. A center tap it: is provided for the plate inductance, and the positive plate supply lead it is connected thereto. The radio frequency is bypassed to ground through the anode condenser ii. A convenient output coil i8 is coupled tothe plate inductance It. A negative anode supply lead 19 is directly grounded through the negative ground lead 29, and a suitable cathode bias resistor 2! is inserted between the negative anode lead l9 and the cathode lead 8. A grid bias resistor 22 is connected in series with the negative anode supply lead is, and the grounded end of the grid inductance 5 23. A relay armature 2d is positioned to connect a pair of contacts 25, which are connected to the opposite ends of the cathode bias resistor 2i.

Practically all tuned-plate tuned-grid oscillators, when the tuned-grid through a relay coil circuit is set at the same frequency as the tuned-plate circuit, will oscillate by virtue of the grid-to-plate capacity inside the tube. Our circuit, under the same conditions, will oscillate, but we prefer, in order to properly utilize our keyed circuit, to stop it from oscillating by careful adjustment of the neutralizing condenser It. We then prefer to utilize a series keying circuit comprising a plate keying inductance 26, coupled to the plate inductance I 0, and a grid keying inductance 2T coupled to the grid inductance 5. couplings may be fairly loose and variable to control the degree of oscillation. A key 28 is inserted in one of the leads between the two inductances 26 and 21. The circuit couplings are adjusted so that when the key 28 is closed the tube will oscillate at the desired degree, the positive feed-back through inductances Z5 and. TT

overcoming the negative feed-back introduced by the neutralizing condenser I4. The use of the key 28 to' make and break the feed-back current starts and stops oscillation of the tube. It is of course obvious that the keying device itself may bein any of the forms well' known in the art.

In the circuit we have described, the grid has a constant bias thereon through the resistor 22, but the cathode may be also biased by the res'istor 2| so that the effective bias of the grid, with respect to the cathode, is a function of the difference between the biases given by the grid bias resistor 22 and the cathode bias resistor 2!. The relay armature 24 is so set that when the tube is not oscillating and there is no grid current flowing in the relay coil 23, the contacts 25-25 are closed, thus shorting resistor 2| and placing a reduced efiective bias on the grid when the key 28 is up. When the key 28 is depressed the tube starts to oscillate and take grid current, this grid current flowing through relay coil 23 pulls up the armature 24 and opens the short across the cathode bias resistor 2|, allowing the full bias of the grid bias resistor 22 plus that of the cathode bias resistor 2| to be impressed between the grid 3 and the cathode 2. Thus there is placed upon the tube a relatively high bias while the tube is oscillating, but the bias is reduced when the tube is not oscillating, allowing the nonoscillating plate current to be consider- Both of these ably higher than it would be if the bias had not been changed. We prefer to adjust the relative biases given by the resistors 2| and 22 so that the heat dissipation of the tube I during oscillation is relatively constant, thus eliminating from the circuit one of the factors which tends toward variation of oscillating output.

In the circuit as shown, we also prefer to make the inductance I6 relatively high and the condenser Il small, and contrariwise, make the grid inductance 5 relatively small and the grid tuning condenser 5 large. This again tends to stabilize the normally stable tuned-plate tuned-grid circuit.

It will be obvious from the above description that the tube is under full control of the keying means, and that by the proper adjustment of the neutralizing condenser l4, and the proper coupling of the two feed-back coils to the grid and plate inductances respectively, that the tube, when not oscillating, may be so adjusted as to be just on the verge of oscillation, and that the strength of oscillation may be readily controlled by the feed-back coupling. Under these circumstances the response of the tube to the keying device is practically instantaneous. Current passing through the key is reduced to a minimum, eliminating a large amount of trouble from arcing. Power supply circuits are not broken nor are any of the other adjustments of the tube seriously interfered with.

We claim:

In an oscillator system having athermionic tube, a tuned-grid circuit and a tuned-plate circuit connected thereto and adapted to produce oscillations in said tube by virtue of the internal feedback of said tube, means for neutralizing said feed-back to stop said oscillation, an inductance coupled to said grid circuit, a second inductance coupled to said plate circuit, said inductances being in series, the coupling of said inductances to said circuits being sufiicient to transfer sufficient energy therebetween to overcome the effect of said neutralizing means, and means for interrupting the connection between said inductances at signal frequency.

WILLIAM W. EITEL.

JACK MCCULLOUGH. 

